Questions about Planet Nine
Short answers, pulled from the story.
What is Planet Nine and why do scientists think it exists?
Planet Nine is a hypothetical planet proposed to exist in the outer Solar System, far beyond Neptune, with an estimated mass of roughly 4.4 to 10 times that of Earth. Scientists suspect it exists because a group of extreme trans-Neptunian objects, bodies orbiting the Sun at distances averaging more than 250 AU, have orbits that cluster in the same direction, a pattern that gravitational simulations suggest is best explained by a large unseen planet. Astronomers Konstantin Batygin and Michael Brown calculated in 2016 that there was only a 0.007 percent chance the alignment arose randomly.
Where is Planet Nine located in the Solar System?
Planet Nine has not been directly observed, so its location is uncertain. Estimates of its semi-major axis have ranged from roughly 290 AU to 700 AU depending on the analysis. A 2025 study by Amir Siraj, Christopher F. Chyba, and Scott Tremaine proposed a semi-major axis of 290 AU. Its aphelion, the farthest point from the Sun, is estimated to be in the general direction of the constellation Taurus, and if it is near aphelion it would likely be more than 600 AU from the Sun.
Who proposed the Planet Nine hypothesis?
Konstantin Batygin and Michael Brown of the California Institute of Technology published the Planet Nine hypothesis in early 2016. Brown had previously led the team that discovered many of the trans-Neptunian objects used as evidence, and Batygin developed the dynamical framework showing how a distant massive planet could explain the clustering of those objects' orbits.
How are astronomers searching for Planet Nine?
The primary ongoing search uses the 8-meter Subaru Telescope, which has both the aperture to detect faint objects and a wide field of view. Two teams, led by Batygin and Brown and by Trujillo and Sheppard, are conducting that search, and both expect it to take up to five years. Researchers have also searched archived data from WISE, NEOWISE, Pan-STARRS, the Catalina Sky Survey, the Zwicky Transient Facility, and TESS; data from the Zwicky Transient Facility alone have ruled out 56 percent of possible Planet Nine positions.
What are the main arguments against the Planet Nine hypothesis?
The Outer Solar System Origins Survey documented over 800 trans-Neptunian objects and, after correcting for observational bias, found no evidence of the orbital clustering that Planet Nine is supposed to produce. The Dark Energy Survey independently reached the same conclusion from 316 newly discovered objects. Samantha Lawler, an author on one of those studies, said the hypothesis does not hold up to detailed observations and suggested Neptune's outward migration could explain the extreme orbits without any unknown planet.
Could Planet Nine be a primordial black hole instead of a planet?
In 2019, Jakub Scholtz and James Unwin proposed that a primordial black hole could be responsible for the orbital clustering attributed to Planet Nine. They argued that such an object would be undetectable by reflected light but could produce gamma rays through interactions with surrounding dark matter, detectable by the Fermi LAT. Konstantin Batygin acknowledged the idea is possible but stated there is currently not enough evidence to make it more plausible than other alternatives.